Objective- Patients with CAD have reduced endothelial progenitor cells (EPCs) which may contribute to endothelial dysfunction and cardiovascular risk. We hypothesized that repetitive exercise might increase EPCs with improvement in endothelial function. Methods- Twenty-eight CAD patients participated in a 3 month cardiac rehabilitation program, with 1 hour exercise sessions 3 times weekly; all were on medical management that included statins (previously reported to increase circulating EPCs). Mononuclear cells were isolated from blood and cultured in fibronectin-coated wells, with replating of 1 million nonadherent cells/well at 48 hours to remove from mature endothelial cells in the circulation. EPC colony-forming units (CFU) were identified as cell clusters with endothelial outgrowth at 7 days. Endothelial function was determined by flow-mediated dilation (FMD) of the brachial artery in response to post-ischemic hyperemia (% change from resting diameter). Results- At 3 months, average energy expenditure (METs) per session increased from 14.7+/-1.2 to 19.5+/-1.3 (P less than 0.001 vs. baseline) and treadmill exercise duration (modified Bruce protocol) increased from 14.0+/-0.6 to 15.3+/-0.5 minutes (P less than 0.001 vs. baseline). EPC-CFUs (8 wells counted) increased from 9.6+/-1.7 at baseline to 26.0+/-4.9 at 3 months (P less than 0.01). Sixteen patients showed improved FMD, from 5.1% to 7.7%; their baseline EPC-CFU values, METs per session, improvement in exercise time, and changes in weight and in lipid, glucose and C-reactive protein levels over 3 months were no different than patients without improved FMD. However, patients with improved FMD had substantially greater EPC-CFU at 3 months (35.8+/-7.4 vs. 13.1+/-3.3, P equals 0.002) and greater increases in EPC-CFU from baseline (27.3+/-7.5 vs. 3.1+/-4.1, P less than 0.01) compared with non-responders. For the entire group, improvement in FMD correlated significantly with increase in EPC-CFU (P less than 0.05). Conclusions- Cardiac rehabilitation benefits exercise performance in CAD participants. However, improvement in endothelial function - and by extension, cardiovascular risk - may be dependent on mobilization or activation of progenitor cells with endothelial differentiation potential.